I have a HPI K5.9 engine and have it in a savage 25 chassis with a two speed transmission in it … I was wondering if I have to get a three speed transmission to put in it so that it can run or will the 2 speed work with it

The 2 speed is actually stronger than the 3 speed for the most part, so you will be fine running the 5.9 in it.

It will work, but wont be too quick. 3spd is just as strong as the 2spd with a 10$ mod, 3spd is actually stronger after . End result is the exact same first gear, more tq in 2nd, faster speed in 3rd & a stronger transmission overall.

2spd vs 3spd

forget-a-bout-it

Okay will the 3 speed tranny fit in the 25 chassis??

Yup, 3spd & 2spd fit in the same box. Newer boxes the top pops off for easy access. you want to run the new box if you put in the 3spd and both boxes mount up to the TVPs the same.

It wont be slow by any means.... The 3rd gear is really not much higher ratio than the regular 2nd gear anyway. It'll make a difference, but it wont be a lot... The 2 speed IS stronger stock for stock.